Initiation process



2 E Q m 1 2 1 CROSS REFERENCE SEARCH R0031 March 17, 1970 D. CLEAVERETAL 3,501,675

mmzmon PROCESS L Filed. Sept. 12, 1967 INVENTORS pews cLEAvER PETER aMAwoMw US. Cl. 3151l1 19 Claims ABSTRACT OF THE DISCLOSURE A method ofinitiating and maintaining a hot gaseous plasma by striking an arcbetween two electrodes in the gas stream-to establish a plasma in thegas and subsequently removing the electrodes while maintaining a plasmaby the passage of an oscillatory current around the walls of the tubethrough which the gas is flowing. The invention also includes apparatusfor initiating and maintaining a plasma comprising a pair of electrodespositioned in a tube and moveable therefrom and inlet means fof a gasand an electrically-conductive element surrounding a portion of the tubein the region of the plasma.

The present invention relates to an improved method for the initiationof electrically induced gaseous plasmas Which is particularly suitablefor, but is not limited to, inductively heating a gas by means of anelectrical current oscillating at a frequency below about 1 megacycle/second.

Hitherto, the initiation of gaseous plasmas inductively heated by meansof an oscillating current of a frequency below about 1 megacycle/secondhas proved extremely difficult.

One method by which such a plasma may be initiated is described inNature 211 pp. 841842 (1966). This method requires the provision of twogenerators, one of which must be capable of producing an oscillatingcurrent having a frequency in excess of about 1 megacycle/ second ofappropriate power, for example at least about 1 kilowatt, and the otherof which produces an oscillating current at the lower frequencies atwhich it is desired to maintain the plasma.

'The output of the generator of the higher frequency current is thenconnected to the ends of a number of turns of an electrical conductorwound around a. gasconfining tube and the output of the generator of thelower frequency current is coupled to the ends of a number of turns ofan electric conductor wound around the gas-confining tube downstream (inthe direction of plas- Ina-forming gas flow).

The plasma-forming gas is passed through the gasconfining tube; thegenerators are switched on and a plasma is initiated beneath the turnsof conductor coupled to the higher frequency generator by conventionalmeans, for example by the insertion of a rod of conducting material togive the required ion density or by the use of a Tesla coil. The flow ofgas is then increased thereby causing the plasma to move down thegas-confining tube until it is established under the turns of theconductor coupled to the lower frequency current. The latter is adjustedto the desired value, if necessary, to maintain theplasma and the higherfrequency generator switched otf.

It will be appreciated that such a method of initiation is expensive,since, inter alia, it requires the presence of two generators, istime-consuming and requires considerable care in operation to achievethe desired results.

' It is an object of the present invention to provide an improved methodfor initiating and maintaining an inductively-coupled gaseous plasmawhich is particularly suited for use with oscillating currents having afrequency below about 1 megacycle/ second.

According to the present invention a method for initiating the formationof a hot plasma in a plasma-forming gas comprises passing the gasthrough a gas-confining tube provided with two electrodes, supplying theelectrodes with an electric current while passing an oscillating currentcapable of maintaining inductively a plasma in the gas around thegas-confining tube, forming an arc between the electrodes therebyestablishing a plasma in the gas and thereafter disconnecting andwithdrawing the electrodes while inductively maintaining the plasma bymeans of the oscillating current passing around the gasconfining tube.

According to the present invention also apparatus for the initiation andmaintenance of a hot plasma in a plasma-forming gas comprises agas-confining tube, inlet means for said gas, an electrically-conductiveelement surrounding at least a portion of said tube connectable to asource of oscillatory electric current to maintain a plasma in said gaswithin said tube, a pair of electrodes moveable to positions within saidtube and connectable to a source of electric current to generate an arebetween said electrodes in the volume of the tube enclosed by the coiland means to withdraw said electrodes from said volume.

The plasma-forming gas is preferably argon but may, if desired, byanother monatomic gas having a low ionisation potential and low energyrequirement per unit volume to reach ionisation temperature. Examples ofsuch gases are neon and krypton.

The gas-confining tube is preferably of a non-conducting material, forexample silica, but it may be possible to use a tube of anelectrically-conducting material, particularly if such a tube has thecharacteristics described and claimed in our co-pending United Statesapplication 506,109 i.e. a tube having a thin wall which is dividedlongitudinally by electrically insulating material.

The electrodes which are placed within the gas-confining tube arepreferably of carbon. It is preferred not to use metals if metalliccontamination of the gas-confining tube is to be avoided.

The electrodes are normally introduced into each end of thegas-confining tube. It has been found convenient to introduce the upperelectrode through a self-sealing aperture on the underside of thedistribution head for the plasma-forming gas, for example through aspring loaded flap or the like. The lower electrode may be introducedthrough the open end of the gas-confining tube'through which the gasheated by the plasma issues. 1

For ease of operation it has been 'found advisable to provide mechanicalmeans for withdrawing the electrodes from the tube, once the plasma hasbeen established, for example by means of an electric motor attached bycable or the like to the upper electrode and by means of a rod memberor-,the like to the lower electrode.

The supply of an electric current to the electrodes is normally obtainedby connecting the electrodes to the output of the generator which is tosupply the oscillatory current to inductively maintain the plasma.Alternatively the electrodes can be supplied from a separate source suchas a DC. generator. Suitable switching gear is provided to disconnectthe supply of current to the electrodes, when required.

The output of the oscillating current generator is coupled to anelectrically conducting element wound around the gas-confining tube.This element is conveniently a length of copper tube, through which acoolant, for ex 3 ample water, can be passed and the ends of which areadapted. to be connected to the source of oscillating cur rent.

There are suitably at least three and preferably at least five turns ofthe conducting element around the gas confining tube and the firstand/or last turn may be reversed in order to conduct the current in theopposite direction to that of the adjacent coil. This assists inmaintaining the plasma within that part of the gas-confining tubebeneath the turns of the electrically conducting element. Where thefirst and last turns are reversed, there should be at least five turnsin all.

The normal operation according to the process of the present inventionis to introduce the electrodes (the switch in the circuit from theoscillating current generator being in the open position) into thegas-confining tube until the space between them is such that an arc willbe formed between them when the power available is switched on. Theswitch is then closed and the flow of plasma-forming gas is commenced.The generator is then switched on.

An arc is formed between the electrodes and when this is established theelectrodes are withdrawn slowly and the power from the generatorincreased to a value sufficient to ensure that a plasma can beinductively maintained in the gas-confining tube. As the electrodes arewithdrawn the arc becomes attenuated and is finally extinguished byopening the switch. It is simultaneously replaced by an inductivelyheated gaseous plasma.

The electrodes are then finally withdrawn from the gas-confining tube.

When striking and initially maintaining the arc, it has been foundconvenient to supply the electrodes with current at a frequency in therange 100 kilocycles/sec. to 1 megacycle/sec. and particularly in therange 100 kilo cycles/sec. to 300 kilocycles/sec. and a generator powerin the range 30 to 50 kilowatts and to maintain the resulting hot plasmawith currents of these frequencies and generator powers.

It is, however, equally applicable to the initiation and maintenance ofinductively heated plasmas at currents of higher frequency. Such plasmascan, however, be initiated easily by more conventional means and theprocess of the present invention may be unnecessary in such cases.

One device for carrying out the process of the present invention willnow be described by way of example only with reference to thediagrammatic illustration shown in the accompanying drawing.

As shown in the drawing the device consists of a gasconfining tube 1partly encircled by a coil forming a conducting element 2 the ends ofwhich are connected to output terminals 3 and 4 of an oscillatingcurrent generator (not shown) capable of producing an oscillatingcurrent of the desired frequency and power. Provision is also made (butnot shown) for passing a coolant through the electrically conductingelement 2.

Output terminal 3 is connected via switch 5 and flexible lead 6 to anelectrode 7 slidably located axially in the gas-confining tube 1 at itsupper end. Output terminal 4 is connected via a flexible lead 6 toelectrode 8 axially moveable within tube 1 at its lower end.

The electrode 7 is formed of carbon and to the top is attached a cord 9by which the electrode can be withdrawn through an axially alignedaperture fitted with a spring loaded flap 10 in the plasma-forming gasdistribution head 11. The head 11 is fitted with a conduit '12 for thesupply of gas to the gas-confining tube via holes 14. V

The upper part of electrode 8 is carbon and is mounte on a brass rod 13.This electrode may be moved manually upwardly and downwardly as desired.

The following example shows one method of carrying out the process ofthe invention.

Example An apparatus as described above was set up in which thegas-confining tube 1 was formed of silica and had an internal diameterof 2 inches and a length of 14 inches. The gas distribution head 11 wasformed of brass and was provided with 4 holes 14 each having a diameterof inch equidistantly spaced around the circumference.

The carbon electrodes each had a diameter of 0.5 inch.

5 /2 turns of 7 inch diameter copper tube formed the coil around thegas-confining tube and cooling water was passed through this at a rateof 4litres/minute.

Argon was supplied to the distribution head through conduit 12 at a rateof 70 litres/ minute.

With the switch 5 in the open position, the position of the electrodeswas adjusted so that a gap of /s inch was left between them and thegenerator adjusted to provide a current at a frequency of 200kilocycles/second.

The switch 5 was closed and the generator started, thus forming an arebetween the electrodes 7 and 8. The electrodes 7 and 8 were then slowlywithdrawn from each other and the plate power of the generator wasincreased to 30 kilowatts at a frequency of 200 kilocycles/ second.

Finally, the arc was broken by opening the switch 5 and the inductivelycoupled plasma was thereby initiated and maintained. The electrodes 7and 8 were withdrawn from the gas-confining tube 1 (thereby allowing thespring loaded flap 10 through which the electrode 7 had passed toclose).

The hot gaseous plasma was thereafter maintained without difiiculty.

What is claimed is:

1. A method of initiating the formation of a hot plasma in aplasma-forming gas which comprises passing the gas through agas-confining tube provided with two electrodes, supplying theelectrodes with an oscillating electric current while passing anoscillating electric current capable of maintaining inductively a plasmain the gas around the gas-confining tube, said oscillating electriccurrent having a frequency of kilocycles per second to 1 megacycle persecond and a generator power of from 30 to 50 kilowatts, forming an arcbetween the electrodes and withdrawing and disconnecting the electrodesto break the arc and to establish a plasma in the gas, the plasma beinginductively maintained by means of the oscillating current passingaround the gas-confining tube.

2. A method according to claim 1 in which the frequency is from 100kilocycles per second to 300 kilocycles Per second.

3. A method according to claim 1 in which after formation of an arebetween the electrodes the power of the oscillating current is increasedto a value sufficient to maintain the plasma while the electrodes arewithdrawn from the tube and prior to extinguishing the arc.

4. A method according to claim 1 in which the gas is a monatomie gas.

5. A method according to claim 4 in which the mom atomic gas is argon.

6. A method according to claim 1 in which the electrodes are introducedone at each end of the gas-confining tube.

7. Apparatus for the initiation and maintenance of a hot plasma in aplasma-forming gas comprising a gasconfining tube, inlet means for saidgas, an electricallyconductive element surrounding at least a portion ofsaid tube connectable to a source of oscillatory electric current tomaintain a plasma in said gas within said tube, a pair of electrodesmoveable to positions within said tube and connectable to a source ofelectric current to generate an arc between said electrodes in thevolume of said tube enclosed by the element and means to withdraw saidelectrodes from said volume.

8. Apparatus according to claim 7 in which the elec trodes are formed ofcarbon.

fi. Apparatus according to claim 7 in which the elec trodes areintroduced one at each end of said tube which is mounted substantiallyvertically.

10 Apparatus according to claim 7 in which the gas confining tube isfitted with a gas-distribution head at. one end having an axiallyaligned aperture fitted with sealing means adjacent the interior of thetube.

11. Apparatus according to claim 10 in which one of said electrodes ismoveable within said aperture to extend into said tube.

12. Apparatus according to claim 10 in which the scal ing means is aspring loaded flap.

13. Apparatus according to claim 11 in which the means to withdraw theelectrode comprises a cable.

14, Apparatus according to claim 11 in which the other electrode isintroduced into the tube at the other end and is carried by a moveablerod member.

15. Apparatus according to claim 7 in which the electrically-conductiveelement is a coil wound round a portion of the gas-confining tube.

16. Apparatus according to claim 15 in which the .coil is formed ofcopper tube.

17. Apparatus according to claim 16 in which the coil has at least threeturns around the gas-confining tube.

18. Apparatus according to claim 17l'in which the coil has at least fiveturns around the gas-confining tube.

19, Apparatus according to claim 7 in which the source of oscillatoryelectric current is connected through switch means to the electrodes andalso to said electricallyconductive element.

References Cited UNITED STATES PATENTS 3,277,265 10/ 1966 Reboux 2l9l21X 3,296,410 l/1967 Hedger 313231 X 3,324,334 6/1967 Reed 3l5-111 XRAYMOND F. HOSSFELD, Primary Examiner US. Cl. X.R.

